The objective of this research is the characterization of phosphoproteins which appear in cells transformed by Rous sarcoma virus. We propose to determine whether these phosphoproteins result from phosphorylation by the protein product (pp60src) of the viral transformation (src) gene, and whether they may mediate other events in cellular transformation. The experimental approach involves the analysis of the kinetics of phosphorylation of cellular polypeptides in cells infected by temperature-sensitive mutants of the virus, and a comparison, using two-dimensional gel electrophoresis, of the polypeptides phosphorylated in vivo with those phosphorylated in vitro.